This invention relates to an enhanced off recovery process and in particular, a process for enhancing the recovery of oil through fie simultaneous introduction of a non-condensible miscible gas, e.g., carbon dioxide, and an aqueous drive fluid, e.g., brine.
Many petroleum producing formations require assistance to economically produce hydrocarbons therefrom. In particular, the use of primary production techniques, i.e., the use of only the initial formation energy to recover the crude oil, typically recovers less that 50 % Of the original oil present in the formation. Even after the secondary technique of waterflooding, a significant portion of the oil remains behind.
To solve this problem, the art has looked to the use of certain enhanced oil recovery (EOR) techniques. These techniques can be generally classified as thermally based recovery methods, i.e., utilizing steam, or gas-drive based methods that can be operated under either miscible or immiscible conditions. That is, in the gas-drive based methods, for certain crude oils and formation temperatures, the gases, which are typically non-condensible, become miscible with the oil above a pressure known as the minimum miscibility pressure. Above this pressure, these non-condensible gases attain a supercritical state having characteristics of both gases and liquids.
However, because the viscosity of these non-condensible fluids, such as carbon dioxide, is significantly less than that of the crude oil present within the reservoir (carbon dioxide has only 5 to 10% of the viscosity of, e.g., light oil), significant channelling of the gas typically occurs and as a result much, if not most, of the oil in the reservoir is nonetheless bypassed by the gas. In particular, because of the differences in viscosity, "breakthrough" of the carbon dioxide occurs and the subsequently injected gas preferentially follow the path of the breakthrough, thereby resulting in poor sweep efficiencies in the reservoir.
One technique for decreasing this mobility of miscible CO.sub.2 in the reservoir involves the use of a drive fluid, i.e., a higher viscosity fluid used to "push" the slug of carbon dioxide. One example of such a process is the Water Alternating-Gas (WAG) process. Such a process comprises the alternating introduction of a non-condensible, miscible gas such as carbon dioxide, nitrogen, methane, mixtures of methane with ethane, propane, butane and higher homologues, with an aqueous drive fluid, e.g., brine. In this process, the water serves to prevent the mobile non-condensible gas from channelling directly from an injection well to a producing well. This ability to slow down gas movement through the reservoir is capable of providing improved contact between the gas and the oil remaining in the reservoir.
However, this process is not without its own set of problems. For example, the gas and the aqueous drive fluid may not be distributed uniformly within the reservoir. In particular, due to differences in viscosity between the water and the carbon dioxide, "gravity override", i.e., gravity segregation of the components, can occur, thereby decreasing the effectiveness of the recovery process. In addition, the WAG process faces certain economic limitations, both with respect to the cost of the non-condensible gas and the labor intensive nature of certain aspects of the process, e.g., alternating between the two components.
Thus, the need still exists for an improved process for the enhanced recovery of petroleum employing a non-condensible, miscible gas.